It does look like there's anomalous behavior here of some sort, but I'd expect --test=-- to result in test="--", myself, rather than an error. My intuition is that '--' would need to be preceded by a space to function as the 'end of options' marker. Because of that, I've never tried the above scenario with a unix command, so maybe my intuition is wrong :)
Just for reference (the code paths may be different), there is another open issue about -- parsing, issue 13922.

I just tried this with grep's "-e" and "--regexp":
$ cat > temp.txt
a--b
cdef
$ grep -e-- -v temp.txt
cdef
$ grep --regexp=-- -v temp.txt
cdef
$ grep -e -- -v temp.txt
cdef
$ grep --regexp -- -v temp.txt
cdef
And with diff's "-I" and "--ignore-matching-lines":
$ cat > temp2.txt
cdef
a--b
$ diff temp.txt temp2.txt
1d0
< a--b
2a2
> a--b
$ diff -I-- temp.txt temp2.txt
$ diff -I -- temp.txt temp2.txt
$ diff --ignore-matching-lines -- temp.txt temp2.txt
$ diff --ignore-matching-lines=-- temp.txt temp2.txt
$
Note though that for options that don't take an argument, the "--" is just removed:
$ grep -v -- a temp.txt
cdef
$ diff -i -- temp.txt temp2.txt
1d0
< a--b
2a2
> a--b
So I guess the unix rule is: if an option that takes an argument is followed by "--", use that as the option's argument and continue parsing as usual. If an option that takes no argument is followed by "--", then delete the "--" and treat all following flags as positional arguments.
Argparse can't follow this directly, because then people who are using "--" to signal the end of an option with nargs="*" would start getting "--" included in that list. (And I know people have used "--" this way for a while.)
I guess my preference is what R. David Murray suggests: "--" when part of an argument (i.e. not separated by spaces) is treated like any other characters, and only a lone "--" signals the end of options (and is ignored otherwise). That would mean that both "--test=--" and "-t--" in your example would give you ["--"], and the other errors would stay as you saw them.